Desalination of silver halide photographic emulsions utilizing sulfated polyvinyl alcohol

ABSTRACT

Sulfated polyvinyl alcohol is used as a coagulating agent to precipitate gelatin-silver halide photographic emulsions in order to remove undesired dissolved salts.

Muted S1a1es Patent [1 1 1 3,867,154 Lukeian Feb. 18, 1975 [5 DESALINATION OF SILVER HALIDE 2,376,371 5/1945 Lowe et a1. 96/114 X PII TOGRAPIIIC EMULSIONS UTILIZING games W114 enyon e a SULFATED POLYVINYL ALCOHOL 2,759,914 3/1956 Kenyon et all 260/913 X [75] Inventor: Joseph Lukeian, Millburn, NJ.

[731 Assign: Kennel & E5595 Company Primary Examiner-N0rman G. Torchin Momstown, Assistant ExaminerA. T. Suro Pico 22 Filed; July 23 1973 Attorney, Agent, or FirmLionel N. White, Esq [2]] Appl. No.: 381,650

' 57 ABSTRACT [52] US. Cl 96/94 R, 96/114, 96/1 14.7 [51] Int. Cl G03c 1/02 s lf d polyvinyl alcohol is used as a coagulating [58] Flew of Search 96/94 114-7, 114-8 agent to precipitate gelatin-silver halide photographic emulsions in order to remove undesired dissolved [56] References Cited Salts UNITED STATES PATENTS 2,376,005 5/1945 Potter et a1. 96/1 14 4 Claims, N0 Drawings BACKGROUND Techniques for precipitating, or coagulating, the gelatin-silver halide components of photographic emulsions has for the most part displaced the noodle and wash method as a means of, removing dissolved salts from the emulsion composition. In contrast to the involved time-consuming procedures and careful control of conditions required. in the noodling method, the ad dition of organic solvents or inorganic salts with acidification have proved to be particularly acceptable in their simplicity and general economy of time and active materials. Although use of these materials has substantially reduced the consumption of washing waters as compared with the noodling process, thereremains a requirement for substantial sources of purified water, as well as the ever-increasing problems of solvent recovery and effluent disposal.

More recently, use has been made of yet more effective coagulating agents, such. as phthalated gelatin, which have to some extent shortened processing times and alleviated the recovery and disposal problems. Similarly, the conservation of wash water has been improved through the use of polymeric coagulating agents, such as the sulfonated polyvinyl acetals described in US. Pat. No. 2,772,165. These latter materials, however, suffer from their susceptibility to hydrolysis which results in the generation of aldehydes from which they are derived, particularly where the photographic emulsion is stabilized in the acid pH range. The tanning effect of such aldehyde compounds on the gelatin matrices of photographic compositions is generally harmful, such as by the inhibition of development, and is of notable disadvantage in so-called wash off imaging products.

While such previous coagulating agents have served to reduce some handling problems, their use leads to the formation of finely-divided granular curd precipitates as a result of which significant-losses of silver from photographic emulsion compositions as experienced during washing operations.

SUMMARY According to the present invention, the advantages of precipitation of gelatin-silver halide emulsions for the removal of undesired salts, as achieved with previous polymeric coagulating agents, are retained, yet the coagulation is more thorough and efficient when a sulfated polyvinyl alcohol is employed as the coagulating material, thus substantially eliminating a cause of physical loss of silver. The addition to a newly-prepared gelatin-silver halide photographic emulsion of as little as of a'sulfated polyvinyl alcohol, based on the weight of gelatin in the silver halide-forming composition, with acidification to reduce the pH of the composition to about 3.0, effects a rapid formation and setting of cohesive coagulant curds of the gelatin-silver halide mass of the composition. So effective is the agglomeration of these components that decantation of the remaining liquor followed by a single clean water rinseremoves substantially all the undesired dissolved salts from the composition, yet little silver-bearing coagulate is lost in the decanting operation. Reconstitution of the photographic emulsion can readily be accomplished by redispersion of the gelatinous precipitate at a pH in excess of about 6.0 and incorporation, if desired, of additional gelatin, stabilizing salts, and other commonlyused addenda.

DESCRIPTION The sulfated polyvinyl alcohol coagulating agent of the present invention exhibits a substantial increase in water solubility as compared with; the polyvinyl alcohol from which it is derived, and is readily prepared in accordance with the following procedures.

EXAMPLE .I

A solution of 44 gm. of polyvinyl alcohol (degree of hydrolysis: 99%) was prepared in 500 ml. of dimethylformamide and the solution was cooled to about 5C. An additional 500 ml. of dimethylformamide was added to the solution and the gelatinous mass was maintained between about 0C and 5C in a salt/ice bath. During a period of about 2 /2 hours, 40 gm. of sulfur trioxide in the vapor state was added with stirring EXAMPLE 111 Following the general procedure: set forth in Example 1, another product sulfated to 12.4%, indicating a sulfation of about 20% of the available hydroxyl groups, was

prepared by the addition of 29 gm. of to the polyvinyl alcohol/dimethylformamide solution. I

The sulfated polyvinyl alcohol prepared in Example I was used in the manner set forth below in the desalination of a common gelatin-silver halide emulsion. Its use effected not only a rapid coagulation and precipitation of a gelatin-silver halide composition which was to a comparatively great extent free from dissolved salts, but the cohesive consistency of theprecipitated curd was such as to enable an almost immediate decantation of the supernatant liquor with no observable carryover of curd material. This result was particularly notable in comparison with the results obtained through the use of commonly-employed prior art coagulants; e.g., phthalated gelatin or ammonium sulfate.

As a ready measure of the amount of dissolved salts contained in a newly-prepared gelatin-silver halide composition, reference is conveniently made to the electrical conductivity of the composition. In this manner the decrease in conductivity, and thus the reduction in dissolved salt content, may be followed through successive washings, if such are required, after precipitation of the gelatin-silver halide mixture. Washing can be discontinued at any desired salt level. With a common photographic composition as utilized in the present examples, experience has demonstrated that washing to a conductivity level of about 3,000 to 6,000 mhos is indicative of a dissolved salt level which is acceptable in (little deleterious) the ultimate photographic composition. A measure of the conductivity of an emulsion composition during a coagulation and washing desalination operation thus provides a simple means for evaluating the efficacy of a particular coagulating agent.

EXAMPLE III The gelatin-silver halide emulsion utilized in the evaluation of the coagulating agents of the present invention was prepared in the following manner:

Ten gm. of photographic gelatin (high bloom) were swollen and dissolved in 520 ml. of water at about 50C. To the gelatin solution was added a solution of 4.2 gm. of silver nitrate in about 200ml. of water (36C) was added to the gelatin solution and the composition was completed by the addition of a solution of 19 gm. ofNaCl and 1.3 gm. KBrin about 170 ml. ofwater.

The emulsion thus formed was acidified to a pH of about 3.0 by the addition of sulphuric acid, 20%. There was then added to the composition 1.5 gm. of the sulfated polyvinyl alcohol of Example I dissolved in 20 ml. of water. There followed immediately the rapid formation of a cohesive gelatinous curd. The supernatant liquid was decanted and the curd was washed with 1000 ml. of water. The single wash reduced the original conductivity of'the' emulsion from about 22,000 mhos, to about 2,200 mhos, indicating an extremely effective removal of the soluble salts. The composition was washed a second time with 1,000 ml. of water and the resulting conductivity measured about 1,800 mhos, indicating a substantially complete removal of dissolved salts in the initial washing operation. Despite the two washes, a silver analysis indicated a loss of only 1.01% of the calculated silver content of the original emulsion.

EXAMPLE IV As a comparison in the determination of the efficacy of the use of the sulfated polyvinyl alcohol according to the present invention, desalination of a gelatin-silver halide emulsion prepared according to Example III was effected utilizing ammonium sulfate, often employed in the prior art as a coagulating agent. To the emulsion, at a pH of about 3.0, was added 150 ml. ofa 50% aqueous solution of ammonium sulfate. After about 2 to 3 minutes of stirring, a finely-divided curd was formed. This curd was allowed to settle for an additional 20 minutes and the supernatant liquid decanted. Despite the care exercised during the decantation, an appreciable amount of fine curd-was carried over. The precipitate was washed and decanted twice using about 1,000 ml. of water in each wash. The final wash water indicated a conductivity of about 6,400 mhos. The silver analysis of the resulting emulsion indicated a silver loss of about 1 1%. 1

EXAMPLE V Phthalated galatin was substituted for the photo- EXAMPLE VII To a gelatin-silver halide composition prepared asin Example III and adjusted to a pH of about 3.0 was added a solution of 1.2 gm. of the sulfated polyvinyl alcohol of Example I in 20 ml. of water. Precipitation and one washing reduced the conductivity of the emulsion from about 22,000 to about 3,600 mhos. A solution of 44 gm. of gelatin in 1,000 ml. of water was added to the washed curds and the emulsion heated to about 50C. Addition of 20 ml. ofa 0.5% KI solution and 13 ml. of 7.5% HCl were added to final the emulsion and a coating was prepared at a coating weight of about 13.5 mg. Ag per square decimeter. The coated material exhibited normal shelf life, and after exposure and development yielded an image density of about 2.5.

What is claimed is:

1. In the process of preparing a photographic silver halide composition which comprises forming a fluid emulsion comprising silver halide and gelatin, treating said emulsion to form a coagulate of silver halide and gelatin components thereof, separating said coagulate and the supernatant fluid vehicle of said emulsion, washing said coagulate, and redispersing said silver halide and gelatin components in a fluid vehicle, the improvement wherein said treatingcomprises the steps, in either order, of:

a. acidifying said emulsion to a pH of about 3.0; and

b. dissolving in said emulsion a sufficient amount of a sulfated polyvinyl alcohol to effect said coagulate formation.

2. The improvement according to claim 1 wherein said polyvinyl alcohol has a sulfur content of between about 12.4% and 17. 5%.

3. The improvement according to-claim 1 wherein the amount of said sulfated polyvinyl alcohol dissolved in said emulsion is at least about 10% by weight of said gelatin component.

4. The improvement according to claim 1 wherein said polyvinyl alcohol comprises a 99% hydrolyzed product sulfated at between about 20% and 50% of the hydroxyl groups. 

1. IN THE PROCESS OF PREPARING A PHOTOGRAPHIC SILVER HALIDE COMPOSITION WHICH COMPRISES FORMING A FLUID EMULSION COMPRISING SILVER HALIDE AND GELATIN, TREATING SAID EMULSION TO FORM A COAGULATE OF SILVER HALIDE AND GELATIN COMPONENTS THEREOF, SEPARATING SAID COAGULATE AND THE SUPERNANT FLUID VEHICLE OF SAID EMULSION, WASHING SAID COAGULATE, AND REDISPERSING SAID SILVER HALIDE AND GELATIN COMPONENTS IN A FLUID VEHICLE, THE IMPROVEMENT WHEREIN SAID TREATING COMPRISES THE STEPS, IN EITHER ORDER, OF: A. ACIDIFYING SAID EMULSION TO A PH OF ABOUT 3.0; AND B. DISSOLVING IN SAID EMULSION OF SUFFICIENT AMOUNT OF A SULFATED POLYVINYL ALCOHOL TO EFFECT SAID COAGULATE FORMATION.
 2. The improvement according to claim 1 wherein said polyvinyl alcohol has a sulfur content of between about 12.4% and 17.5%.
 3. The improvement according to claim 1 wherein the amount of said sulfated polyvinyl alcohol dissolved in said emulsion is at least about 10% by weight of said gelatin component.
 4. The improvement according to claim 1 wherein said polyvinyl alcohol comprises a 99% hydrolyzed product sulfated at between about 20% and 50% of the hydroxyl groups. 